Stack read out-of-bounds in parse_sha1_header_extended using git 2.10.0

Hi,

We found a stack read out-of-bounds parsing object files using git 2.10.0. It was tested on ArchLinux x86_64. To reproduce, first recompile git with ASAN support and then execute:

$ git init ; mkdir -p .git/objects/b2 ; printf 'x' > .git/objects/b2/93584ddd61af21260be75ee9f73e9d53f08cd0

Finally you can trigger the bug using several commands from git (other commands that parses all objects will work too), for instance:

$ git fsck

The ASAN report is here:

==2763==ERROR: AddressSanitizer: stack-buffer-overflow on address 0x7ffe16e4a690 at pc 0x0000006fe5dc bp 0x7ffe16e4a530 sp 0x7ffe16e4a520
READ of size 1 at 0x7ffe16e4a690 thread T0
    #0 0x6fe5db in parse_sha1_header_extended /home/g/Work/Code/git-2.10.0/sha1_file.c:1684
    #1 0x702cd4 in sha1_loose_object_info /home/g/Work/Code/git-2.10.0/sha1_file.c:2660
    #2 0x70332c in sha1_object_info_extended /home/g/Work/Code/git-2.10.0/sha1_file.c:2696
    #3 0x7038e0 in sha1_object_info /home/g/Work/Code/git-2.10.0/sha1_file.c:2745
    #4 0x648498 in parse_object /home/g/Work/Code/git-2.10.0/object.c:260
    #5 0x48d46d in fsck_sha1 builtin/fsck.c:367
    #6 0x48da47 in fsck_loose builtin/fsck.c:493
    #7 0x707514 in for_each_file_in_obj_subdir /home/g/Work/Code/git-2.10.0/sha1_file.c:3477
    #8 0x70775b in for_each_loose_file_in_objdir_buf /home/g/Work/Code/git-2.10.0/sha1_file.c:3512
    #9 0x707885 in for_each_loose_file_in_objdir /home/g/Work/Code/git-2.10.0/sha1_file.c:3532
    #10 0x48dc1d in fsck_object_dir builtin/fsck.c:521
    #11 0x48e2e6 in cmd_fsck builtin/fsck.c:644
    #12 0x407a8f in run_builtin /home/g/Work/Code/git-2.10.0/git.c:352
    #13 0x407e35 in handle_builtin /home/g/Work/Code/git-2.10.0/git.c:539
    #14 0x408175 in run_argv /home/g/Work/Code/git-2.10.0/git.c:593
    #15 0x408458 in cmd_main /home/g/Work/Code/git-2.10.0/git.c:665
    #16 0x53fc70 in main /home/g/Work/Code/git-2.10.0/common-main.c:40
    #17 0x7f0f46d43290 in __libc_start_main (/usr/lib/libc.so.6+0x20290)
    #18 0x405209 in _start (/home/g/Work/Code/git-2.10.0/git+0x405209)

Address 0x7ffe16e4a690 is located in stack of thread T0 at offset 192 in frame
    #0 0x702834 in sha1_loose_object_info /home/g/Work/Code/git-2.10.0/sha1_file.c:2614

  This frame has 5 object(s):
    [32, 40) 'mapsize'
    [96, 120) 'hdrbuf'
    [160, 192) 'hdr' <== Memory access at offset 192 overflows this variable
    [224, 368) 'st'
    [416, 576) 'stream'
HINT: this may be a false positive if your program uses some custom stack unwind mechanism or swapcontext
      (longjmp and C++ exceptions *are* supported)
SUMMARY: AddressSanitizer: stack-buffer-overflow /home/g/Work/Code/git-2.10.0/sha1_file.c:1684 in parse_sha1_header_extended
Shadow bytes around the buggy address:
  0x100042dc1480: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x100042dc1490: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x100042dc14a0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x100042dc14b0: 00 00 00 00 00 00 00 00 00 00 f1 f1 f1 f1 00 f4
  0x100042dc14c0: f4 f4 f2 f2 f2 f2 00 00 00 f4 f2 f2 f2 f2 00 00
=>0x100042dc14d0: 00 00[f2]f2 f2 f2 00 00 00 00 00 00 00 00 00 00
  0x100042dc14e0: 00 00 00 00 00 00 00 00 f4 f4 f2 f2 f2 f2 00 00
  0x100042dc14f0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x100042dc1500: 00 00 f3 f3 f3 f3 00 00 00 00 00 00 00 00 00 00
  0x100042dc1510: 00 00 00 00 f1 f1 f1 f1 04 f4 f4 f4 f2 f2 f2 f2
  0x100042dc1520: 00 00 00 00 00 f4 f4 f4 f3 f3 f3 f3 00 00 00 00
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07 
  Heap left redzone:       fa
  Heap right redzone:      fb
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack partial redzone:   f4
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
  Left alloca redzone:     ca
  Right alloca redzone:    cb

Regards, Gustavo.